Linear motors are used in a variety of electrical devices. For example, linear motors are used in exposure apparatuses for semiconductor processing, other semiconductor processing equipment, elevators, electric razors, machine tools, machines, inspection machines, and disk drives.
Exposure apparatuses for semiconductor processing are commonly used to transfer images from a reticle onto a semiconductor wafer. A typical exposure apparatus utilizes one or more linear motors to precisely position a wafer stage holding the semiconductor wafer relative to the reticle. The images transferred onto the wafer from the reticle are extremely small. Accordingly, the precise positioning of the wafer and the reticle is critical to the manufacturing of the wafer. In order to obtain precise relative positioning, the reticle and the wafer are constantly monitored by a metrology system. Subsequently, with the information from the metrology system, the reticle and/or wafer are moved by one or more linear motors to obtain relative alignment.
One type of linear motor is a shaft type linear motor. A typical, shaft-type linear motor includes a magnet array that generates a magnetic field and a coil array that encircles the magnet array. The coil array includes a plurality of coils that are individually supplied with an electrical current. The electrical current supplied to the coils generates an electromagnetic field that interacts with the magnetic field of the magnet array. This causes the coil array to move relative to the magnet array. When the coil array is secured to the wafer stage, the wafer stage moves in concert with the coil array.
Unfortunately, the electrical current supplied to the coils also generates heat, due to resistance in the coils. Most linear motors are not actively cooled. Thus, the heat from the coils is subsequently transferred to the surrounding environment, including the air surrounding the linear motor and the other components positioned near the linear motor, The heat changes the index of refraction of the surrounding air. This reduces the accuracy of any metrology system and degrades machine positioning accuracy. Further, the heat causes expansion of the other components of the device. This further degrades the accuracy of the device. Moreover, the resistance of the coils increases as temperature increases. This exacerbates the heating problem and reduces the performance and life of the motor.
In light of the above, it is an object of the present invention to provide a system for maintaining an outside surface of a linear motor at a set temperature during operation. It is another object of the present invention to provide a system for cooling the coil array of a shaft-type linear motor. Still another object of the present invention is to provide an exposure apparatus capable of manufacturing high density semiconductor wafers.